A broad variety of cryptographic techniques and corresponding apparatus have been developed to meet increasing needs for secure communications among civilian and military users. These needs are fostered by increasingly stringent security requirements for industrial, financial, government, police, subscriber media, and other commercial and civil endeavors wherein unauthorized data interception would cause harm to the public good or to private business interests.
Typical encrypted data includes computer-based records, telephone conversations and other voice data, telemetry, facsimile transmissions, earth-satellite and satellite-satellite communications from a variety of sources including the Global Positioning System, and subscriber information distribution systems. Motivations for encrypting data include improved signal to noise ratio by promoting a more even data mix (e.g., transmitting an encrypted, balanced mixture of "ones" and "zeroes" rather than an unbalanced, non-encrypted string of "ones" or "zeroes"), enforcement of subscription fees, privacy of privileged conversation, national security concerns, and maintenance of financial transaction integrity for both prevention of computer-based crimes and to provide convincing assurance of confidentiality and user authorization.
Improved sophistication in intelligence surveillance and code-breaking methods have motivated the creation of new ciphering algorithms and corresponding equipments. By way of example and not intended to be limiting, several classes of cryptographic methods currently in broad use include the Data Encryption Standard (DES), as described in Federal Information Processing Standards Publications FIPS 46-1, "Data Encryption Standard," and FIPS 81, "DES Modes of Operation," both published by the United States Department of Commerce, Public Key cryptographic techniques such as that taught in "Fast Real-Time Public Key Cryptography," U.S. Pat. No. 4,399,323, to Paul S. Henry, and "Multiple-Destinational Cryptosystem for Broadcast Networks," U.S. Pat. No. 4,365,110, to Lin-Nan Lee and Shyue-Ching Lu. The above-identified U. S. Patents are included herein by reference.
An element common to these diverse cryptographic devices and algorithms is the need for authorized senders and receivers to share cipher key information of some form enabling encryption and subsequent decryption of the intended message.
A disadvantage of prior art public-key cryptographic systems for secure communications such as telephony is that public-key data encryption and decryption is a very slow process. Accordingly, public-key cryptography is often used to transmit small amounts of information, such as a traffic key for a more rapid cryptographic technique, in a secure fashion over a public transmission medium, such as a radio channel or a telephone line.
Encryption methods for real-time bidirectional communication include a variety of ciphering techniques such as those disclosed in FIPS 46-1 and 81, supra, and other techniques developed by companies supplying such apparatus. These methods are capable of providing excellent data integrity provided that both parties have or have access to suitable traffic keys for data ciphering. Typically these methods operate on blocks of digital data formed from input data of varying digital or analog form.
Sometimes multiple communication channels must be used, as for example when more than a single sender and receiver are involved, either serially or in parallel. Frequently, not all users have identical equipment or identical cryptographic keys or algorithms. Under these circumstances, redundant equipments differing in cipher keys or cryptographic algorithms and devices may well be needed at each sending or receiving site. This results in increased communication equipment needs and expenses. Further, where the type of communications equipment must be mobile or portable, the power requirements, weight, and large size necessary to accommodate multiple, separate secure communications systems are unacceptable.
Even when a single communications terminal accommodates multiple ciphering techniques, user knowledge of the other party's capabilities and manual user selection of suitable secure communications apparatus may be required. This can compromise communications security by increasing user knowledge of the details of the security algorithm and hardware employed, by spreading of authorized user information over a broader number of individuals, and by increasing user involvement in the detailed arrangements required to initiate secure communication. This increases the risk of error in effecting secure communications links.
What is needed is a means for rapid real-time secure communications which accommodates multiple ciphering algorithms and cipher keys in a single apparatus, so that authorized, self-synchronizing communications can be established and maintained between diverse parties. It is further desirable that the apparatus be compact, light weight and have low power requirements.